1. Field of the Invention:
This invention relates to earth boring bits used in the oil, gas and mining industries, especially those having rolling cutters and features to prevent the cutter teeth from "balling up" with compacted cuttings from the earth.
2. Background Information:
Howard R. Hughes invented a drill bit with rolling cones used for drilling oil and gas wells, calling it a "rock bit" because it drilled from the outset with astonishing ease through the hard cap rock that overlaid the producing formation in the Spindletop Field near Beaumont, Tex. His bit was an instant success, said by some the most important invention that made rotary drilling for oil and gas commerically feasible the world over (U.S. Pat. No. 930,759, "Drill", Aug. 10, 1909). More than any other, this invention transformed the economies of Texas and the United States into energy producing giants. But the invention was not perfect.
Mr. Hughes' bit demolished rock with impressive speed, but it struggled in the soft formations such as the shales around Beaumont, Texas, and in the Gulf Coast of the United States. Shale cuttings sometimes compacted between the teeth of the "Hughes" bit, until it could no longer penetrate the earth. When pulled to the surface, the bit was often, as the drillers said, "balled up" with shale--sometimes until the cutters could no longer turn. Even moderate balling up slowed the drilling rate and caused generations of concern within Hughes' and competitive engineering organizations.
Creative and laborious efforts ensued for decades to solve the problem of bits "balling-up" in the softer formations, as reflected in the prior art patents. Impressive improvements resulted, including a bit with interfitting or intermeshing teeth in which circumferential rows of teeth on one cutter rotate through opposed circumferential grooves, and between rows of teeth, on another cutter. It provided open space on both sides of the inner row teeth and on the inside of the heel teeth. Material generated between adjacent teeth in the same row was displaced into the open grooves, which were cleaned by the intermeshing rows of teeth. It was said, and demonstrated during drilling, " . . . the teeth will act to clear each other of adhering material." (Scott, U. S. Pat. No. 1,480,014, "Self-Cleaning Roller Drill", Jan. 8, 1924.) This invention led to a two cone bit made by " . . . cutting the teeth in circumferential rows spaced widely apart . . ." This bit included" . . . a series of long sharp chisels which do not dull for long periods." The cutters were true rolling cones with intermeshing rows of teeth, and one cutter lacked a heel row. The self cleaning effect of intermeshing thus extended across the entire bit, a feature that would resist the tendency of the cutters becoming balled-up between rows of teeth in soft formations. (Scott, U.S. Pat. No. 1,647,753, "Drill Cutter", Nov. 1, 1927.)
Interfitting teeth were shown for the first time on a three cone bit in U.S. Pat. No. 1,983,316. The most significant improvement being the width of the grooves between teeth, which were twice as wide as those on the two cone stucture without increasing uncut bottom. This design also combines narrow interfitting inner row teeth with wide non-interfitting heel rows.
A further improvement in the design is shown in U.S. Pat. No. 2,333,746, in which the widest heel teeth were partially delected, a feature that decreased balling and enhanced penetration rate. A refinement of the design was the replacement of the narrow inner row teeth with fewer rows of wider teeth, which again improved performance in shale drilling.
By now the basic design of the three cone bit was set: (1) All cones had intermeshing inner rows, (2) the first cone had a heel row and a wide space or groove equivalent to the width of two rows between it and the first inner row with intermeshing teeth to keep it clean, (3) a second cone had a heel row and a narrow space or groove equivalent to the width of a single row between it and the first inner row without intermeshing teeth, and (4) a third cone had a heel and first inner row in a closely spaced, staggered arrangement. A shortcoming of this design is the fact that it still leaves the most critical outer portion of the cutting structure subject to balling.
Another technique of cleaning the teeth of cuttings involved flushing drilling fluid or mud directly against the cutters and teeth from nozzles in the bit body. Attention focused on the best arrangement of nozzles and the direction of impingement of fluid against the teeth. Here, divergent views appeared, one inventor wanting fluid from the nozzles to " . . . discharge in a direction approximately parallel with the taper of the cone" (Sherman, U.S. Pat. No. 2,104,823, "Cutter Flushing Device", Jan. 11, 1938), while another wanted drilling fluid discharged ". . . approximately perpendicular to the base [heel] teeth of the cutter." (Payne, U.S. Pat. No. 2,192,693, "Wash Pipe", Mar. 5, 1940.)
A development concluded after World War II seemed for a while to solve the old and recurrent problem of bit balling. A research effort of Humble Oil and Refining Company resulted in the "jet" bit. This bit was designed for use with high pressure pumps and bits with nozzles that pointed high velocity drilling fluid directly against the borehole bottom, with energy seemingly sufficient to quickly disperse shale cuttings, and simultaneously, keep the cutters from balling up because of the resulting turbulent flow. This development not only contributed to the reduction of bit balling, but also addressed another important phenomenon which became later known as chip holddown.
From almost the beginning, Hughes and his engineers recognized variances between the drilling phenomena experienced under atmospheric condition and those encountered deep in the earth. Rock at the bottom of a liquid-filled borehole is much more difficult to drill than the same rock brought to the surface of the earth. Model sized drilling simulators showed in the 1950's that removal of cuttings from the borehole bottom is impeded by the filter cake resulting from the use of drilling mud. ("Laboratory Study of Effect of Overburden, Formation and Mud Column Pressures on Drilling Rate of Permeable Formations", by R. A. Cunningham and J. G. Eenick, Journal of Petroleum Technology, Jan. 1959). A thin layer of this filter cake is benefical to seal and stabilize the borehole. But if there is a large difference between the borehole and formation pressure, also known as overbalance or differential pressure, this layer of filter cake sometimes increases in thickness with the addition of cuttings from the bottom and forms a strong mash layer, which keeps the cutter teeth from reaching virgin rock. The problem is accentuated in deeper holes. One approach to overcome this perplexing problem is the use of ever higher jet velocities in an attempt to blast through the filter cake and dislodge cuttings so they may be flushed through the well bore to the surface.
The filter cake problem and the balling up problem are distinct since filter cake build-up occurs mostly in permeable formations such as sand, and balling is typically seen when drilling impermeable shales. Yet, these problems can overlap in the same well since both formations have to be drilled by the same bit. Inventors have not always made clear which of these problems they are addressing, at least not in their patents. However, a successful jet arrangement must deal with both problems; it must clean the cones but also impinge on bottom to overcome chip holddown.
The direction of the jet stream and the area of impact on the cutters and borehole bottom receives periodic attention of inventors. Some interesting, if unsuccessful, approaches are disclosed in the patents. One patent provides a bit that discharges a tangential jet that sweeps into the bottom corner of the hole, follows a radial jet, and includes an upwardly directed jet to better sweep cuttings up the borehole. (Williams, Jr., U.S. Pat. No. 3,144,087, "Drill Bit With Tangential Jet", Aug. 11, 1964.) The cutters have unusual cutter arrangement, including one with no heel row of teeth, and two of the cutters do not engage the wall of the borehole. One nozzle extends through the center of the cutter and bearing shaft and another exits at the bottom of the "arm" (also sometimes called incongruently the "leg" of the bit body) near the corner of the borehole.
There is some advantage to placing the nozzles as close as possible to the bottom of the borehole. (Feenstra, U.S. Pat. No. 3,363,706, "Bit With Extended Jet Nozzles", Jan. 16, 1968.) The prior art also shows examples of efforts to orient the jet stream from the nozzles such that they partially or tangentially strike the cutters and then the borehole bottom at an angle ahead of the cutters. (Childers, et al, U.S. Pat. No. 4,516,642, "Drill Bit Having Angled Nozzles For Improved Bit and Well Bore Cleaning", May 14, 1985.)
In spite of the extensive efforts of inventors laboring in the rock bit art since 1909, including those of the earliest, Howard R. Hughes, the ancient problem of rock bits "balling up" persists. The solutions of the past have reduced balling in many drilling environments, and the bit that balls up so badly that the cutters will no longer turn is a species of the problem that has all but completely disappeared. Now, the problem is confined to only part of the rock bit and often escapes detection. It is more subtle and disguised by other events in the drilling process and largely unappreciated as a cause of poor drilling performance. However, it is present and can reduce penetration rates to one-half in typical shale drilling. It gains added importance in view of conservative estimates that more than one-half of all drilling takes place in ductile and balling shales.